A paint primarily forms a coating film on a substrate for preventing corrosion. A paint which has enhanced corrosion preventive properties is specifically called "corrosion preventive paint" and is used for many usages.
Corrosion preventive properties of the corrosion preventive paint are generally attained by a material which generates a chromate ion. The chromate ion has two functions. One is an oxidizer function whereby the chromate ion (CrO.sub.4.sup.2-) is reacted with a metal atom, such as Fe to form stable gamma-Fe.sub.2 O.sub.3, and the other is a deposition function whereby the chromate ion is reduced to form a chromium oxide, for example Cr.sub.2 O.sub.3 which is deposited on a metal surface. According to these two functions, a physical barrier film is formed on the metal surface to develop excellent corrosion preventive properties.
However, the chromium (VI) having high corrosion preventive properties is toxic to human body and therefore very limited in use by several regulations in Japan and many other countries. Accordingly, many efforts have been payed to develop a corrosion preventive agent which is not or little toxic to human body. For example, phosphate materials, especially zinc phosphate, silicophosphate, aluminum condensed phosphate and the like are now studied and some of them are commercially available. The phosphate materials, however, have only the deposition function whereby the phosphate materials are reacted with metal to produce a matSSial protecting the metal substrate, but does not have the oxidizer function. The phosphate materials have low corrosion preventive properties in comparison with the above mentioned chromate ions.